This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Chronic abdominal pain (CAP) is an extremely pervasive childhood condition and, like IBS in adults, it is one of the functional bowel disorders without a clear framework of understanding or an effective treatment. However, new research suggests that small intestinal bacterial overgrowth (SIBO) may be the unifying pathophysiology that explains the variety of symptoms experienced by patients with IBS. As CAP in children is believed to be a precursor to IBS in adults, we hypothesize that children with this disorder have a significantly greater prevalence of small intestinal bacterial overgrowth (SIBO) than normal, healthy children, and that eradication of bacterial overgrowth with antibiotics will reduce symptoms of chronic abdominal pain in children with this condition. To prove this, we will first aim to determine the prevalence of SIBO in both healthy children and those with CAP. We will do this by performing a lactulose breath hydrogen test, the gold standard for the noninvasive measurement of SIBO, on 40 healthy controls and 80 subjects with CAP. We will then assess whether eradication of SIBO with antibiotics will reduce symptoms of chronic abdominal pain in children with this condition. To do this we will randomize, in a double-blinded fashion, the 80 CAP patients to receive a 10-day course of either the antibiotic Rifaximin or a placebo. After completion of the treatment we will evaluate all these patients for eradication of bacterial overgrowth by repeating a lactulose breath hydrogen test. We will also assess for symptom improvement by re-administering questionnaires.